The invention relates in general to an apparatus for sensing vehicle wheel angular speed and, in particular, to such an apparatus for use in an antilock brake control system.
Vehicle antilock braking systems respond to signals representing the angular velocity of one or more wheels of the vehicle. Typically, a wheel speed sensor includes at least one magnet mounted on a stationery portion of the vehicle and a toothed or slotted metal rotor which rotates in response to the wheel movement and which is located adjacent the sensing device. As the rotor teeth or slots pass the sensing device, variations in the magnetic flux produce a voltage the frequency of which is a function of the angular velocity of the wheel and the number of teeth or slots in the rotor.
In order to generate a voltage level which provides an accurate indication of the speed, the sensing device and the rotor must be accurately positioned with respect to one another during initial assembly and they must remain in this relationship even after extended periods of operation in which the components are subjected to road shocks and repeated vibrations. Therefore, most wheel speed sensors are mounted in the area of the wheel. However, such a location requires that additional care be taken not to damage the sensor during wheel maintenance.
As an alternative, the rotor can be directly mounted on the axle shaft and the sensing device mounted on the differential housing of a drive axle. However, such a mounting location also creates other problems. During initial installation of the axle, the axle shaft must be inserted into the differential through the interior of the drive axle housing. Thus, great potential exists for damaging the sensor either during installation or removal for maintenance. Furthermore, since the end of the drive axle housing adjacent the wheel has an inside diameter only slightly larger than the outside diameter of the axle shaft, the size of the rotor is limited if it is to be mounted on the drive axle before assembly to the differential.
Another problem exists where better braking control and shorter stopping distances are desired. Such system performance demands a higher number of teeth on the rotor for better resolution. Thus, the distance between the sensing device and the rotor must be minimized and maintained within predetermined limits throughout the rotor rotation. The normal manufacturing tolerances for an axle shaft and the side gears of the differential tend to militate against the maintenance of the proper spacing or "air gap".